Final answer:
The towel's temperature increases due to absorption of sunlight and the conduction of heat from the warm sand. It partially loses heat through emission of infrared radiation and convection currents of warm air rising off of it.
Step-by-step explanation:
When you leave a towel out on the beach on a hot, sunny day, the towel's temperature will change due to a combination of heat transfer mechanisms. First, absorption of sunlight will cause the towel's temperature to increase as it absorbs the energy from the sun's rays through a process called radiation. This is because the towel acts like an ideal black-body radiator that converts electromagnetic waves from the sun into heat. Secondly, the towel will emit infrared radiation, which is the release of absorbed heat in the form of electromagnetic waves. This emission will enable the towel to lose some of the gained heat back to the environment.
In addition to radiation, conduction of heat will alter the towel's temperature as the warm sand underneath the towel will transfer heat to the towel via direct contact. Lastly, convection currents in the towel may slightly impact its temperature. While the towel itself does not have currents within it, the air above the towel, warmed by the absorbed heat, can rise and be replaced by cooler air. This movement of air can carry away some of the heat from the towel, allowing for heat loss through convection.